DESCRIPTION: (Applicant's Abstract) The conversion of nucleosides to their corresponding nucleotides is pivotal for both the growth and treatment of tumor cells. It is evident that maintenance of normal levels of endogenous nucleotides is essential to prevent cell cycle arrest and the induction of apoptosis, while the ability of a tumor cell to convert nucleoside analogs to their corresponding triphosphates is essential for a chemotherapeutic response. Steady state levels of nucleotide analogs are achieved and maintained to a large extent by the relative rates of nucleoside phosphorylation by nucleoside kinases and degradation by nucleotidases. Although considerable progress has been made over the past five years in the elucidation of the nucleoside kinases important in drug metabolism and chemotherapeutic sensitivity, far less is known about the nucleotidases and their roles in both normal cellular metabolism and drug resistance. In addition, very little is known about the role of intracellular localization of these enzymes as potential modulators of the interaction and activities. The applicant proposes to address two areas of nucleoside metabolism in tumor cells that will help to elucidate these issues. First, she will characterize the activities of each of the three major 5' nucleotidases that have been identified. She will determine the relationship of ecto-5'-nucleotidase expression to multidrug resistance, characterize a new cytosolic nucleotidase (cN-I) with broad specificity that appears very likely to play an important role in drug resistance, and compare its activities to those of the high Km cytosolic nucleotidase (cN-II). In addition, she will determine whether a second isoform of the high Km enzyme exists in human cells, as it does in two other species. As a second new initiative, she will define the role of adenosine kinase in tumor cells and determine whether its association with cytoskeletal elements is important to its function. She will also ask whether post-translational modifications or protein-protein interactions of AK are important to its role in regulating cellular adenine nucleotide pools under stress conditions. Finally, she will determine whether deficiency of adenosine kinase and the resulting decrease in adenosine phosphorylation sensitizes cells to the effects of adenine nucleotide depletion under conditions such as hypoxia and glucose deprivation. These studies should extend our knowledge of the metabolism of nucleosides and nucleotides into several new areas and provide the basis for a greater understanding of the mechanisms and consequences of variability in enzyme expression for malignant cells.